Review fixes: MyST markup, language, and grammar

[rkratky]

Summary

Review fixes for PR canonical#136 (Add a tutorial for implementing RAG with Spring AI).

Changes

MyST markup:

• Added two blank lines above each heading (except the first)
• Added one blank line above/below all code blocks, lists, admonitions, and images
• Converted all terminal CLI commands to the {terminal} directive (sphinx-terminal)
• Converted all code blocks to {code-block} directive with :caption: for file-named blocks
• Converted error message blocks to {terminal} with :output-only:
• Removed __File__: lines (captions now carry filenames)
• Heading anchor (unblocking-opensearch)= placed directly above heading
• Replaced "section 1.4.4" with {ref}unblocking-opensearch``

Language and grammar:

• Reworded future tense to present simple ("This tutorial will use" -> "This tutorial uses", "will show" -> "shows")
• Reworded "you may" / "may be" / "if" / conditionals to imperative form
• Removed "please" from prose
• Fixed "lets" -> "let's" (2x)
• Fixed "Please give process this" -> "Give this process"
• Fixed "OpenSource" -> "OpenSearch"
• Fixed "is you don't" -> "(skip when already installed)"
• Removed "should" / "would" where expressing uncertainty ("should launch" -> "launches", "should unblock" -> "unblocks", "would reset" -> "resets")

Semantic markup:

• Package names: {pkg}
• File/directory names: {file}
• Commands: {command}

Serial comma:

• Added serial comma to lists of 3+ items
• Did not apply to 2-item lists

Other:

• Ordered lists use 1. for all items